The construction materials of water-cooled nuclear reactors are corroded by the aqueous coolant and small amounts of their constituent elements are released into the coolant. These constituent elements become neutron activated in the reactor core and are ultimately deposited in the form of their oxides on the vessel and pipework surfaces throughout the coolant circuits, giving rise to large radiation dose rates in the circuit. It is desirable to remove these surface oxide layers to reduce the radiation dose rates prior to man access.
Water systems containing oxide scales (e.g. boilers) have traditionally been cleaned using acids and complexing reagents. These reagents depend on an elevated hydrogen ion concentration [H.sup.+] to attack and destabilise the metal oxide and sometimes employ a complexing agent to increase the thermodynamic solubility of the resulting metal ions. An example of this type of reagent is a mixture of oxalic and citric acids. This type of reagent will dissolve the surface oxide layers formed in the coolant circuits of water-cooled nuclear reactors when used at high concentrations in the range of from 3 to 6% by weight. However, the volumes of coolant circuits are large and this cleaning process gives rise to unacceptably large volumes of radioactive waste. If it were possible to reduce the reagent concentration, then both the reagent and the dissolved radioactive compounds could be removed by ion exchange giving rise to a low-volume, solid waste which can be easily handled. The maximum feasible concentration of reagent for such a process is generally about 0.1% by weight. At this concentration the traditional acidic cleaning reagents dissolve the surface oxide layers in water-cooled reactors unacceptably slowly and an alternative reagent is required.
The major ionic exchange constituent of all surface oxide layers occurring in the circulating coolant of a water-cooled nuclear reactor is Fe.sup.III (see Table 1 hereinbelow for details of these oxides and their compositions).
TABLE 1 ______________________________________ Ionic Composition of Typical Nuclear Reactor Deposited Oxides Ionic Constituents of Oxides Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 Fe.sub.2 NiO.sub.4 Fe.sub.2.25 Cr.sub.0.15 Ni.sub.0.6 O.sub.4 ______________________________________ Cr.sup.3+ 0 0 0 0.15 Ni.sup.2+ 0 0 1.0 0.60 Fe.sup.2+ 0 1.0 0 0.40 Fe.sup.3+ 2.0 2.0 2.0 1.85 ______________________________________
Fe.sup.3+ is the major metal ion in these water reactor deposited oxides (.sup.60 Co and .sup.58 Co, the major sources of radiation dose, are chemically insignificant).
Because it is well known that Fe.sup.3+ can be reduced to Fe.sup.2+, previous attempts to improve the performance of acids or chelating reagents have included the addition of reducing agents. In principle such reagents can increase the rate of dissolution of oxides containing Fe.sup.3+ by the transfer of an electron thereto, converting this ion into Fe.sup.2+ and thereby destabilising the lattice. Although Fe.sup.3+ is the major constituent of the surface oxide layers, also Cr.sup.3+ may also be contained in these oxides and is reduced to Cr.sup.2+.
We have discovered that not all reducing agents attack the surface oxide layers rapidly, the most effective being those that can transfer a single electron since each Fe.sup.III can only readily accept one electron. Of the one-electron reducing agents only those which provide an easy pathway for electron transfer dissolve the surface oxide layers rapidly and examples of these are the oxidation states of certain transition metals. The expression low oxidation state is used herein to imply an oxidation state lower than that which is normally stable with respect to oxidation by air in aqueous solution. We have further discovered that, in order for such a reagent to react with these oxides and cause rapid dissolution, the conditions must be such that a suitable mechanism for the electron transfer exists. We have discovered that rapid dissolution of Fe.sup.III oxides by low oxidation state transition metal ions can occur either in strongly acidic solutions, or alternatively in weakly acidic solutions, when the reducing metal ion is present in the form of a complex with a suitable chelating ligand. In strongly acidic solutions, the reaction can be promoted by ions which are capable of acting as bridging ligands in an inner-sphere mechanism, by forming a direct link between the Fe.sup.III and the reducing agent; chloride is a typical example of such an ion. Such ions are corrosive to the materials of nuclear reactors and other metals, and strong acids in general are also corrosive. However, we have discovered that complexes formed between chelating ligands and transition metal ions can react rapidly to dissolve Fe.sup.III oxides, using an outer-sphere mechanism, in which no direct link is required between oxide and reducing agent. It is well known that such complexes are usually most stable in weakly acidic or neutral solutions. Thus such reagents can be effective under conditions which are not intrinsically harmful to the material to be cleaned.
The complex reducing agent is formed by the combination of a low oxidation-state transition metal ion and one or more chelating ligands; these two components must be selected such that the complex so formed is both a sufficiently strong reducing agent and capable of rapid electron transfer. It is well known that complexes formed between transition metal ions and chelating ligands often have properties very different from those of the metal ions alone. The conditions must also be carefully chosen such that the desired complex is formed, and such that the rate of electron transfer is sufficiently high. In particular, the pH must be neither too low, which would prevent the formation of the complex, nor too high which can slow down the electron-transfer reaction and can also lead to the precipitation of certain metal ions as hydroxides. The choice of chelating agent and conditions must ensure that all the metal ions dissolved in the decontamination process remain in solution, by forming complexes with the ligand. The oxidised product of the decontaminating agent must also remain in solution as a stable complex. It will be clear to those skilled in the art that the conditions, such as the pH and the concentration of ligand, required to satisfy these criteria will depend on the choice of low oxidation-state metal ions and complexing agent. The present invention is based on these findings.